There are a number of different types of semiconductor-based imager devices, including those employing charge coupled devices (CCDs), charge injection devices (CIDs), hybrid focal plane arrays, and complementary metal oxide semiconductor (CMOS) pixel arrays. Current applications of solid-state imager devices include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, automatic focus systems, star trackers, motion detector systems, image stabilization systems, and other image acquisition and processing systems.
CMOS imager devices are well known. CMOS imager devices are discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31 (12), pp. 2046-2050 (1996); Mendis et al., “CMOS Active Pixel Imager devices,” IEEE Transactions on Electron Devices, Vol. 41 (3), pp. 452-453 (1994); and are also disclosed in U.S. Pat. Nos. 6,140,630, 6,204,524, 6,310,366 and 6,326,652, assigned to Aptina Imaging Corporation, the entire disclosures of which are incorporated herein by reference.
CMOS imager devices are typically formed with an array of pixels each containing a photosensor, such as a photogate, phototransistor, photoconductor, or photodiode. The photosensor in each pixel absorbs incident radiation of a particular wavelength (e.g., optical photons or x-rays) and produces an electrical signal corresponding to the intensity of light impinging on that pixel when an optical image is focused on the pixel array. For example, the magnitude of the electrical signal produced by each pixel can be proportional to the amount of incident light captured. The electrical signals from all pixels are then processed to provide information about the captured optical image for storage, printing, transmission, display, or other usage.
Imager devices have traditionally been constructed so that light impinges on the frontside of a device, the frontside being the top of a device built on a semiconductor substrate. New advances, however, have produced backside illuminated imager devices, which receive incident radiation through a backside of the device substrate. Example methods of forming backside illuminated imager devices are disclosed in U.S. Pat. No. 6,429,036 and U.S. application Ser. Nos. 12/480,440 and 12/533,709 assigned to Aptina Imaging Corporation, the entire disclosures of which are incorporated herein by reference. Backside illuminated imager devices are advantageous because the backside of an imager device has less processing features (metalization, polysilicon, diffusions, etc.) that would obscure photosensors and result in a loss of photons reaching the sensitive area of the photosensors.
Backside illuminated imager devices presently require that electrical connections be formed to metal patterns that are below the substrate (the substrate being at the top of these devices). It is possible to remove portions of the substrate to provide connections to electrical pads below the substrate (using wire bonds, for example), but this results in additional features on the color filter array (CFA) side of the substrate and an uneven topography of the substrate. The uneven topography presents problems with subsequent processing steps. Accordingly, a method to form electrical connections to bond pads that results in minimal topographic features and minimizes processing steps is desired.